Then I found a really cool set of bookcases that almost fit the top of the stairs perfectly. They worked for several years (added bonus - our record collection fit perfectly in the bottom cabinet), but then they too became overcrowded with books. I constantly looked at the bookcases and thought, "Man, if they just were about 18 inches wider, we would have enough space."
I then attached the front veneer to each shelf, and then sanded, stained, and varnished all of the pieces.
To attach the shelves, the top, bottom, and one middle shelf are screwed into the shelves through the existing shelves. The remaining shelves are all attached using black brackets, which I think created a cool look. Finally, the top trim was attached using pocket holes.
The only thing left to decide is where to build the next bookcase...